Radiosurgery and radiotherapy systems are radiation treatment systems that use external radiation beams to treat pathological anatomies (e.g., tumors, lesions, vascular malformations, nerve disorders, etc.) by delivering a prescribed dose of radiation (e.g., x-rays or gamma rays) to the pathological anatomy while minimizing radiation exposure to surrounding tissue and critical anatomical structures (e.g., the spinal chord). Both radiosurgery and radiotherapy are designed to necrotize the pathological anatomy while sparing healthy tissue and the critical structures. Radiotherapy is characterized by a low radiation dose per treatment, and many treatments (e.g., 30 to 45 days of treatment). Radiosurgery is characterized by a relatively high radiation dose in one, or at most a few, treatments.
In both radiotherapy and radiosurgery, the radiation dose is delivered to the site of the pathological anatomy from multiple angles. As the angle of each radiation beam is different, each beam can intersect a target region occupied by the pathological anatomy, while passing through different regions of healthy tissue on its way to and from the target region. As a result, the cumulative radiation dose in the target region is high and the average radiation dose to healthy tissue and critical structures is low. Radiotherapy and radiosurgery treatment systems can be classified as frame-based or image-guided.
In frame-based radiosurgery and radiotherapy, a rigid and invasive frame is fixed to the patient to immobilize the patient throughout a diagnostic imaging and treatment planning phase, and a subsequent treatment delivery phase. The frame is fixed on the patient during the entire process. Image-guided radiosurgery and radiotherapy (IGR) eliminate the need for invasive frame fixation by tracking and correcting for patient movement during treatment.
Image-guided radiotherapy and radiosurgery systems include gantry-based systems and robotic-based systems. In gantry-based systems, the radiation source is attached to a gantry that moves around a center of rotation (isocenter) in a single plane. Each time a radiation beam is delivered during treatment, the axis of the beam passes through the isocenter. In some gantry-based systems, known as intensity modulated radiation therapy (IMRT) systems, the cross-section of the beam is shaped to conform the beam to the pathological anatomy under treatment. In robotic-based systems, the radiation source is not constrained to a single plane of rotation.
In image-guided systems, patient tracking during treatment is accomplished by registering two-dimensional (2-D) intra-treatment x-ray images of the patient (indicating where the patient is) to 2-D reference projections of one or more pre-treatment three-dimensional (3-D) volume studies of the patient (indicating where the patient should be to match the treatment plan). The pre-treatment 3-D volume studies may be computed tomography (CT) scans, magnetic resonance imaging (MRI) scans, positron emission tomography (PET) scans or the like.
The reference projections (reference images), known as digitally reconstructed radiographs (DRRs) are generated using ray-tracing algorithms that replicate the geometry of the intra-treatment x-ray imaging system to produce images that have the same scale as the intra-treatment x-ray images. Typically, the intra-treatment x-ray system is stereoscopic, producing images of the patient from two different points of view (e.g., orthogonal views).
As x-ray imaging technology advances, the sensitivity of the x-ray detectors used to capture the intra-treatment x-ray images is increasing. These increases are due, at least in part, to improved imaging materials (e.g., amorphous silicon) and image capture technologies (e.g., CCD and CMOS imaging arrays) and processing algorithms which reduce the quantum noise and electronic noise levels of the x-ray detectors and increase the signal-to-noise ratios of the intra-treatment x-ray images for any given imaging radiation level. Generally, a higher signal-to-noise ratio produces higher quality images that translate to improvements in image registration and patient tracking due to improved detectability of anatomical features and/or fiducial markers. For any given noise figure, the detectability of an anatomical object can be improved by changing x-ray properties. Two such changes can involve increasing the imaging radiation dose or energy to increase the SNR. FIG. 1 illustrates the improved detectability of an anatomical object 10 in a field of view 20 as the SNR is increased from 1:1 to 2:1 to 5:1 as the radiation dose is increased. X-ray sources used to generate intra-treatment x-ray images are typically set to dose and energy levels sufficient to penetrate larger patients and provide the required x-ray image quality (SNR level) for consistent and reliable tracking of patient and anatomical motion during setup and treatment. However, above a certain minimum SNR (e.g., 1:1), improvements in patient tracking and image registration may be offset by increased risks to the patient from higher radiation doses.